The NSIS script incorrectly passes the default installation dir as
argument for the auto-uninstaller of previous installs.
This is supposed to avoid moving the uninstaller to a temp file, but it
is unclear why that was done, as this leaves the uninstaller behind.
More importantly, now that we have changed the default installation
directory, the auto-uninstall tries to remove the previous install from
the new location, which does nothing and leaves behind several GB on the
disk.